1. Field of the Invention
The present invention relates to a wire bonding apparatus and more particularly to an ultrasonic horn for a bonding apparatus.
2. Prior Art
As seen from FIGS. 7A and 7B, a typical ultrasonic horn which is referred to by the reference numeral 1 and used in a wire bonding apparatus is comprised of a horn main body 3 and a vibrator 4. The horn main body 3 has at one end thereof a capillary 2 through which a wire (not shown) is passed, and the vibrator 4 is screw-connected to another end of the horn main body 3.
The horn main body 3 is formed at its rear end with a flange portion 5 that is fastened to a bonding arm (not shown) of a bonding apparatus.
The vibrator 4 includes a vibration generating source attachment shaft 6, an insulating pipe 7, a vibration generating source 8 and a nut 9. The vibration generating source attachment shaft 6 is screw-connected to the rear end of the horn main body 3. The insulating pipe 7 is fitted over the vibration generating source attachment shaft 6. The vibration generating source 8 is structured by a plurality of doughnut-form electrostriction elements or magnetostriction elements that are fitted over the insulating pipe 7 in a stacked configuration. The nut 9 is screw-connected to the vibration generating source attachment shaft 6 so that the vibration generating source 8 is tightened in place by the nut 9.
A wire bonding apparatus equipped with the above-described ultrasonic horn is described in, for instance, Japanese Patent No. 3128715 (Laid-Open No. H6-196533).
In the above-described ultrasonic horn 1, the vibration of the vibration generating source 8 is transmitted to the entire ultrasonic horn 1 and creates a standing-wave vibration in the ultrasonic horn 1, thus supplying the necessary energy to the capillary 2. In an unloaded state (in which wire bonding is not being performed), the energy accumulates in a stable fashion; and since the ultrasonic horn 1 is designed a precisely and has dimensions in which a node of vibration is at its flange portion 5, there is little vibration in the flange portion and therefore loss of movement of the ultrasonic horn 1 is small even when the ultrasonic horn 1 is attached to the bonding arm (not shown). In the unloaded state, the ultrasonic horn 1 acts in the manner of a tuning fork. The vibration generating source 8 is ordinarily driven by constant-current driving, etc., so that the amplitude has a specified value. When energy is used for wire bonding via the capillary 2, current is applied so that the energy necessary for equilibrium is sent into the vibration generating source 8. In this way, wire bonding that uses ultrasonic waves is performed.
Generally, ultrasonic horns are formed from an iron or steel material. Accordingly, the inertial moment is large; and when the bonding operation is performed at a high speed, the impact load, which is applied when a capillary on the horn contacts the members that are being bonded, such as pellets or leads, increases, resulting in a possibility that the members being bonded are damaged by the capillary.
Japanese Patent Application Laid-Open (Kokai) No. 8-241908, for instance, discloses a conventional wire bonding apparatus that reduces the inertial moment and alleviates the impact load on the members being bonded. In this wire bonding apparatus, the inertial moment is reduced by way of shortening the inter-axial distance between the axial center of the rotating shaft of the ultrasonic horn and the central axis of the ultrasonic horn in the direction of length. In addition, the supporting members that support the ultrasonic horn are formed from a light metal material; as a result, driving of the ultrasonic horn is accomplished by a motor that has a small output as the driving source for the rotating shaft. However, even with such a means as employed in this prior art, there is a limit to the alleviation of the inertial moment and impact load.